Creating bitmaps

A bitmap filled with zero values.
Note: we cannot guarantee the alignment
of the memory block (but typically it is aligned at least to machine word boundary),
but what we can guarantee is that the rows are properly padded.

Blending

Blends two bitmaps with the given weights; that is, the result is
the specified linear combination. If the values are outside the allowed
range (this can happen with the Word8, Word16, Word32 types and weights
whose sum is bigger than 1, or with a negative weight), then they are
clipped. The clipping does not happen with the Float component type.